Hello guys!
Here is another link for freeware that just might work VERY well for your mp4 player. It is called Photo2PocketPC and is meant for..... PocketPC. But I'm thinking it will do wonders for mp4's.
Post back here with results once you've tried it.
It allows you to batch convert whole folders of photos to exactly the right size for your player. Please post results....
Here it is :
http://www.pocketpcfreewares.com/en/index.php?soft=1313
Iridium